Synthesis And Characterization Of NNO-type Tridentate Ruthenium Complexes And Their Application In Acceptorless Dehydrogenative Coupling Reactions

Ruthenium is a rare and multivalent transition metal,which is often used as the catalyst to replace platinum,palladium and other elements in organic synthesis reactions.Due to the 4d⁷5s¹characteristics of the external electron arrangement of ruthenium,ruthenium element often presents a low-valent reduced state or a high-valent oxidation state in different compounds.Therefore,ruthenium catalysts mainly participate in the two major reactions of oxidative dehydrogenation and reductive hydrogenation in organic synthesis.It has important applications in C-H activation,asymmetric synthesis,enol isomerization,etc.In recent years,the pincer complexes have shown excellent characteristics such as high catalytic activity,high stability and high selectivity.Recently,we have prepared a series of NO-bidentate pyridinyl-alcohol-based Ru complexes,which displayed good catalytic efficiency in alcohol oxidation and amide synthesis.To continue the work on phosphine-free metal catalysis,we prepared several NNO-tridentate pyrazolyl-pyridinyl-alcohol ligand and envisioned the novel chelating ligands might offer easy tunability of the electronic and steric properties of the resulting metal complexes via modification of the pyrazolyl ring and pyridine-alcohol motifs.The catalytic performance of these ruthenium pincer complexes in acceptorless dehydrogenative coupling reactions of alcohols was deeply studied,the steric effect of the ligand substituents on the catalytic activity of the complexes was investigated,and the possible catalytic reaction mechanism was discussed.This paper includes the following four parts:1.Four ligands 6-Bppy C(CH₃)₂OH(L1H),6-Bppy C(CH₃)(Ph)OH(L2H),6-Bppy C(Ph)₂OH(L3H),6-Me₂Bppy C(CH₃)₂OH(L4H)were reacted with Ru Cl₃in ethanol,respectively,to obtain four complexes[(L1)₂Ru](1a),[(L2)₂Ru](1b),[(L3)₂Ru](1c)?[(L4)₂Ru](1d).four complexes were characterized by elemental analysis,IR and HR-MS,and their crystal structures were further determined by X-ray single crystal diffraction.The complexes 1a,1c,1d contain two molecular ligands,in which the O atom and metals atoms are connected by a covalent bond,and two N atoms are coordinated with the center metal atom,thus forming a stable five-membered ring and a stable six-membered ring.2.Using the complexes 1a-1d as catalysts,we studied their catalytic activity in the reaction of acceptorless dehydrogenative coupling to form quinolines.furnished the desired products in decent yields of 71%-94%.The complexes 1a-1d showed good catalytic activity in this reaction,and 1a showed the best catalytic activity.respectively.Good yield can be afforded by both electron-donating groups and electron-withdrawing groups.3.Using the complexes 1a-1d as catalysts,we studied their catalytic activity in the reaction of acceptorless dehydrogenative coupling to form quinazolines.the catalytic yield was between 66%-84%.The complexes 1a-1d showed good catalytic activity in this reaction,and 1a showed the best catalytic activity.4.The synthetic mechanism of the synthesis of quinolines and quinazolines by the acceptorless dehydrogenation coupling reaction of alcohols catalyzed by the synthesized ruthenium complexes was explored,and the synthesized complexes were studied in depth through the design of control experiments and synthetic intermediates.The mechanism of catalyzed acceptorless dehydrogenative coupling of2-aminoarylmethanols with ketones and nitriles is reasonably predicted.